Crucibles made of graphite or of silicon nitride but principally of SiO2 (fused silica) are used for melting and for recrystallization of silicon blocks (ingots) of silicon grains, silicon granules or silicon pieces. By means of exactly defined cooling processes, ingots having the desired structures and purities crystallize out from the melt, these ingots are then cut into thin disks and form the active component of photovoltaic systems.
It is essential here that as a result of the materials used in the processing such as crucibles, the solar silicon quality is not negatively influenced and the silicon melts solidify free from defects and can be removed from the crucible undisturbed. In this context, it is important to prevent the corrosive attack of liquid silicon metal on the crucible material since the melt would otherwise be contaminated. Furthermore, adhesions, infiltration and diffusions lead to problems during removal of the ingots so that there is a risk of a cracking or splitting of the polycrystalline silicon block.
As a result of the corrosive silicon melt, the SiO2 crucible is attacked since a chemical reaction takes place between Si and SiO2 to form volatile SiO. In addition, oxygen and undesired impurities from the crucible material enter into the silicon melt in this way.
In particular, adhesions of the solidifying or solidified silicon block should be avoided at all costs since the silicon undergoes very large thermal expansions where the smallest adhesions lead to mechanical stress and thus to rupture of the crystalline structure, which means rejects of silicon material.